Sentinel lymph nodes refer to lymph nodes that directly receive the flow of lymph from the primary lesion of a tumor, and are believed to be the location where lymph node metastasis first occurs. In surgical excision for early-stage cancer, a procedure known as sentinel lymph node navigation surgery is becoming increasingly common. The procedure comprises identifying the sentinel lymph node using a dye or radioactive colloid, excising the lymph node during surgery, examining for the presence or absence of metastasis, and then determining the resection range based on that result. The use of this procedure makes it possible to avoid unnecessary removal of lymph nodes in the case where evidence of metastasis is not found in the sentinel lymph node, thereby reducing the burden on the patient and making it possible to maintain favorable quality of life (QOL).
In order to identify a sentinel lymph node, a dye or radioactive colloid is administered into the periphery of the tumor, and then migration of the dye or radioactive colloid is monitored visually or with a radiation detector. Among conventionally used bioimaging dyes, patent blue, isosulfan blue and indocyanine green are used clinically as blue and green dyes capable of staining sentinel lymph nodes of the greatest importance in terms of the metastatic pathway (M. Kusano, ed., Overview of ICG Fluorescent Navigation Surgery, Intermedica, 2008). However, although imaging methods using patent blue, isosulfan blue or indocyanine green are simple, they diffuse in a relatively short period of time when administered to the body due to their small particle diameter, thereby resulting in difficulties in identification of the true target organ or tissue. Moreover, it is difficult to discriminate between stained lymph ducts and native venous vessels having blue or green color.
Microparticle dispersions containing radioisotopes such as 99 mTc-S nanoparticles can be used for imaging of lymph flow and sentinel lymph nodes (Heiko Schroeder et al., Cancer Metastasis Rev (2006) 25:185-201). However, this approach has the problem of a considerable decrease in detection accuracy (attributable to shine through phenomenon) if the tracer administration site and detected site (such as a lymph node) are in close proximity, as well as the problem of requiring an operator qualified to work in radioactive controlled areas.
Another report revealed that, when liposomes having a particle diameter of 100 nm to 200 nm incorporating fluorescent quantum dots are introduced into a lymph flow, they accumulate in lymph nodes (Maoquan Chu et al., J. Nanopart. Res., DOI 10, 1007/s11051-009-9593-2). Since experiments evaluating the safety and toxicity of using fluorescent quantum particles in the body have not been adequately conducted, and since it is difficult to make accommodations for various particle diameters compatible with various target organs and tissues, bioimaging based on these particles has not been utilized universally in clinical settings.
Indocyanine green (ICG), represented by the following formula, is known to be a near-infrared fluorescent dye that is useful as a fluorescent probe for detecting sentinel lymph nodes.
When a near-infrared fluorescent pigment is used as a dye, sentinel lymph nodes can be identified and distinguished from other tissue by irradiating with excitation light and detecting fluorescence in the near-infrared wavelength region. Moreover, since near-infrared rays are highly permeable in biological tissue, they enable sentinel lymph nodes to be identified at a depth of several centimeters from the skin surface. An example of a method for detecting sentinel lymph nodes using near-infrared fluorescent dye is disclosed in Japanese Patent Application Laid-open No. 2001-299676.
Indocyanine green forms a complex with lipoprotein and generates intense fluorescence in the near-infrared region when excited with light of a specific wavelength (see, for example, Yoneya et al., IOVS 39, 1286-1290, 1998). However, since the binding constant of indocyanine green and lipoprotein is low in comparison with the binding constant with other biomolecules, when a complex of indocyanine green and lipoprotein is introduced into the body, it will rapidly dissociate and can no longer be detected.
An aqueous solution of indocyanine green is reported to be selectively incorporated by rectal cancer when introduced through the anal (Shingo Noura et al., Ann. Surg. Oncol., DOI 10.1245/s10434-009-0711-2). Introduction of aqueous solutions of indocyanine green into organs finds limited applications because its tumor selectivity, diffusibility and retentivity are not consistent but are dependent on the organ where a tumor is located.
Moreover, when attempting to detect a lymph node using an aqueous solution of indocyanine green, the fluorecent signal cannot be monitored for a long time following administration of the dye due to rapid decrease in the fluorescence intensity, thus re-administration of the dye during surgery is often required. In addition, since indocyanine green has a short anchoring time in lymph nodes, it will flow out the lymph duct over time, thereby a fluorescent signal is also generated in lymph nodes farther downstream from the sentinel lymph node. Consequently, it is sometimes difficult to distinguish between the sentinal lymph node and other downstream lymph nodes during surgery.
Thus, there is a need for a fluorescent probe that is able to generate fluorescence of high intensity for a long period of time and have a long retention time in sentinel lymph nodes. Such a prove would allow for identifying sentinel lymph nodes with high accuracy, making it extremely useful in sentinel lymph node navigation surgery.
Reference documents cited in the description are as indicated below. All contents described in these documents are incorporated herein by reference.    Patent Document 1: Japanese Patent Application Laid-open No. 2001-299676    Non-Patent Document 1: Yoneya et al., IOVS 39, 1286-1290, 1998